Aside from Googling (including "fall" or "winter" in the queries), you might see whether the Croatian tourist office has any advice. I assume that public bus service to Plitvice Lakes is cut back (if not eliminated) at that time of year, and I think one-day bus tours are unlikely that late in the season. I believe the Zagreb-Split-Dubrovnik public buses will be running at a decent frequency, but that's just a guess. Zagreb to Split is also possible by train.
Seven days is a rather short time to squeeze in all 4 of your destinations. I'd definitely drop Plitvice (even if it's accessible) if the weather is poor. I was in Zagreb the second week of October 2015, and the weather changed from summer to fall, turning very cool and wet. I assume the temperature will be more moderate along the coast than in Zagreb for your timeframe. But Zagreb is a lively city with good museums to visit and rather short distances to walk. I'd consider it a decent destination in November/December (though I, personally, would never go to Europe that late in the year).
Zadar would be a good stopover on the way from Zagreb to Split.